The increasing demand for electrical power has forced electrical utilities to burn increasing quantities of fossil fuels such as coal; however, electric utilities also face increasing environmental standards imposed upon their operation. In order to reduce air pollutants, electrical utilities have turned to burning low-sulfur coals to fire their boiler furnaces to generate the steam needed for generating electrical power. In addition, electrical utilities generally use a flue gas treatment system to remove the majority of the particulate matter in the gas effluent. A flue gas treatment system generally comprises an electrostatic means, such as an electrostatic precipitator, and a conditioning agent source for introducing a conditioning agent into the flue gas stream to enhance the efficiency of the precipitator in removing particulate matter.
The efficiency of an electrostatic precipitator in removing particulate matter from the boiler flue gas is partially dependent upon the electrical resistivity of the entrained particulate matter in the boiler flue gas. The entrained particulate matter expelled from a boiler fired with low-sulfur coal, i.e., coal having less than 1 percent sulfur, has been found to have a resistivity of approximately 10.sup.13 ohms/cm. It has been determined that the most efficient removal of particulate matter by electrostatic precipitation occurs when the particulate matter resistivity is approximately 10.sup.8 ohms/cm. Therefore, to obtain more effective use of an electrostatic precipitator, the resistivity of the entrained particulate matter from low-sulfur content coals must be reduced. Electrical utilities have long used conditioning agents introduced into the flue gas flow upstream of the electrostatic precipitator to reduce the resistivity of the entrained particles. Various chemicals, such as water and hydrous ammonia, sulfuric acid, sulfur trioxide and phosphoric acid and various ammonia-bearing solutions have been used as conditioning agents.
Flue gas treatment systems which have been considered for use in the generation of sulfur trioxide conditioning agents include those frequently referred to as "slipstream" systems. In theory, some of the flue gas can be removed from the boiler ductwork, passed across a converter where SO.sub.2 contained in the flue gas is converted to SO.sub.3 and piped back into the ductwork, where the SO.sub.3 in the flue gas will reduce the fly ash resistivity and increase precipitator efficiency as described above. An advantage of this method is that the SO.sub.2 and the heat required for the SO.sub.2 /SO.sub.3 conversion are already present in the flue gas at no additional cost.
Systems embodying this theory are known. Most, if not all, suggest removing a small proportion of the flue gas, referred to as a "slipstream", from the duct at a temperature to suit conversion, which is typically approximately 800.degree. F. Because the flue gas contained fly ash, which could inhibit the effectiveness of conventional catalyst material, the slipstream was cleaned by a precipitator because conventional baghouse filters do not normally operate at these temperatures. Precipitators operating at these high temperatures (commonly called "hotside units") were expensive, when fabricated to withstand high temperatures. Following cleaning, the slipstream was then passed across a catalytic converter and the slipstream with an increased SO.sub.3 content was piped back into the ductwork ahead of, or behind, the airheater.
U.S. Pat. No. 3,581,463 discloses such a slipstream method of removing suspended particulate matter from boiler flue gas wherein a portion of the flue gas is withdrawn, electrostatically cleaned to remove particulates, then passed through a catalyst chamber to convert sulfur dioxide contained in the flue gas to sulfur trioxide. The sulfur trioxide is then returned to the main flue gas stream which then passes through an electrostatic precipitator.
The slipstream was usually suggested to be about five percent of the total flue gas for a catalyst efficiency of approximately 80 percent. The slipstream normally contained 400 ppm SO.sub.2 from the combustion of low sulfur coal, and after conversion of the SO.sub.2, it contained about 320 ppm SO.sub.3. When combined with the flue gas, the SO.sub.3 content of such a slipstream could result in about 16 ppm of SO.sub.3 in the flue gas, which could improve the operation of the precipitator.
Such proposed systems frequently included a blower or fan to draw the slipstream through the hotside precipitator and catalytic converter, and insulated probes if the converted slipstream was put back into the ductwork downstream of the airheater. The catalytic converter of such systems was relatively large in order to handle the slipstream at gas velocities suitable for conversion efficiencies of 80 percent or more, and was located close to the economizer outlet ductwork to operate at the correct temperatures.
More recently, Electric Power Research Institute ("EPRI"), Palo Alto, California, has proposed a system in which flue gas is withdrawn from the duct as before, but is passed through a catalytic converter without cleaning. The development of a catalyst which is effective in the presence of fly ash has eliminated the need for cleaning the flue gas. The catalytic converter and associated ductwork of EPRI's system are installed adjacent to the flue gas ductwork. The slipstream is ducted into the catalytic converter near the economizer where gas temperatures are about 800.degree. F. The volume of the slipstream is controlled by an automatic valve. After conversion, the slipstream gas is re-introduced into the flue gas downstream of the airheater via low pressure insulated probes. This EPRI system operates at only a few inches of water gauge pressure, which is the reason for installing the probes downstream of the airheater, and consequently does not require a blower.
A small pilot system installed by EPRI with a capacity of 1,000 acfm at 800.degree. F. has operated without severe problems since November 1990. The converter efficiency of EPRI's system is believed to be approximately 60-80 percent. The catalytic converter is fitted with soot blowers, but catalytic converter pressure drop is so low that they have not been required.
For full scale operation, however, the EPRI slipstream system has some distinct disadvantages. First, the EPRI system requires the development of probes which will work at a low pressure drop to obtain the good SO.sub.3 distribution necessary for effective operation. Unlike conventional probes which normally operate at 3-4 psig, the low pressure probes that are required must work at a gauge pressure of 5 or 6 inches of water column and operate with dust-laden gas. Second, control of the process depends upon the operation of the inlet high-temperature valve and the amount of SO.sub.3 generated is dependent upon the slipstream entry location selected. Third, the SO.sub.3 slipstream must re-enter the ductwork downstream of the airheater. Some plants have short ductwork runs between the airheater and the precipitator, and there is insufficient room for re-entry ductwork. Fourth, the slipstream ductwork must be carefully designed for low pressure drop and be very well insulated, and the slipstream ductwork route may be long and somewhat expensive for some boiler furnace arrangements.
All the proposed slipstream systems known to applicant, including the EPRI pilot plant, propose to use a relatively small slipstream, 2 to 5 percent, and attempt to achieve high-efficiency catalytic conversion before re-introduction into the main flow of flue gas. All of the proposed systems take the slipstream outside of the ductwork for passage through the catalytic converter, and then return the SO.sub.3 -laden gas to the ductwork either upstream or downstream of the airheater. The serious problem attendant with such proposed slipstream systems include not only those set forth above, but also prohibitively high costs and lack of satisfactory process control.
Controlling the flow of a flue gas conditioning agent has been approached in a variety of ways. The quantity of conditioning agent produced has been commonly determined by the quantity of coal being burned, precipitator power and/or the opacity of the flue gas generated by the coal combustion. For example, U.S. Pat. No. 2,864,456 discloses an automatic control for electrostatic precipitators which varies both the electrostatic precipitator voltage and the supply of a conditioning agent, such as water, for conditioning particles to be removed by the electrostatic precipitator and to maintain an optimum sparking rate for maximum efficiency in the removal of the particles.
U.S. Pat. No. 3,523,407 discloses a method of improving the electrostatic precipitation of particles from a flue gas by adding pre-selected amounts of ammonia and water to the flue gas stream upstream of the precipitator.
U.S. Pat. No. 3,665,676 discloses a system to condition the particles of boiler flue gas which includes a metering means for controlling the amount of conditioner injected into the flue gas. The conditioning agent, preferably a salt solution such as ammonium sulfate or ammonium bisulfate, is injected into the flue gas prior to entering the electrostatic precipitator. U.S. Pat. No. 3,665,676 discloses that, if desired, conventional automatic controls can be provided to open the metering means when the flue gas reaches a desired operating temperature or to close it should the temperature fall below the desired operating temperature. In addition, automatic controls can also be made to open the metering means to provide an amount of conditioner needed in proportion to the volume of flue gas to be conditioned.
U.S. Pat. No. 3,689,213 discloses a process for treating flue gas in which gaseous sulfur trioxide is generated in the immediate vicinity of the point of use as required by the quantity of fossil fuel being burned per unit time and is then introduced into the flue gas at a predetermined rate to facilitate the removal of fly ash by an electrostatic precipitator.
U.S. Pat. No. 3,772,178 discloses a system for the production of sulfur trioxide for flue gas conditioning including means to deliver a source of sulfur, such as sulfuric acid, to a vaporizer in proportion to the amount of flue gas generated by the boiler, measured in terms of the electrical output generated at a particular time. As the production of flue gas changes in the boiler system, the proper ratio of sulfuric acid to flue gas is automatically maintained by control means responsive to a signal coming from a boiler capacity index gauge to control the volume of sulfur trioxide being produced by the system.
U.S. Pat. No. 3,993,429 discloses that SO.sub.3 flue gas conditioning systems can operate by sensing the rate of coal combustion and varying the rate of flow of sulfur into the sulfur burner in response to the rate of coal combustion.
U.S. Pat. No. 4,770,674 discloses a system for conditioning flue gas for an electrostatic precipitator, including equipment for converting sulfur into sulfur trioxide. The disclosed systems of U.S. Pat. No. 4,770,674 include a sulfur burner to produce oxidized sulfur, a catalytic converter to convert the oxidized sulfur to sulfur trioxide, and means to control sulfur and air inputs to the sulfur burner. Various inputs to the control means are disclosed, including the outlet temperature of the catalytic converter, and such operating parameters of the exhaust stage of the system as the output temperature of the exhaust gas from the precipitator, the flow rate of the exhaust gas, the power delivered to or the speed of, an induced draft fan, if any, the opacity of the exhaust gas within the stack, and the power dissipated by the precipitator.
U.S. Pat. No. 4,779,207 discloses a system for preconditioning flue gas for electrostatic precipitation that includes a source of an SO.sub.3 conditioning agent, means for controllably adding the conditioning agent to the flue gas, means for detecting the input power level of the electrostatic precipitators, and control means for monitoring the input power level and controlling the amount of conditioning agent added to the gas to maintain the input power to the electrostatic precipitator at predetermined levels.
U.S. Pat. No. 5,011,516 discloses a slipstream method of catalytically oxidizing SO.sub.2 to SO.sub.3 in the presence of fly ash wherein a portion of the flue gas is withdrawn from the main flue gas flow, passed through a converter so that SO.sub.2 within the gas flow is catalytically converted to SO.sub.3, and then the portion flow reinjected into the main flue gas flow upstream from the electrostatic precipitator. The catalytic converter of U.S. Pat. No. 5,011,516 comprises a series of parallel air flow passages which are lined with a catalytic material for converting SO.sub.2 to SO.sub.3. The parallel flow-through passages allow for passage of gasses and particulates without fouling of the catalytic material.
A controller commercially available from Castlet (Electronic Engineers) Ltd., of Lincoln, England, can control an electrostatic precipitator by detecting the presence of deleterious back ionization and intermittently applying voltage to the charging electrodes of the precipitator to minimize the back ionization phenomenon. The Castlet controller detects back ionization by interrupting the applied charging voltage at its peak value and comparing, after a preset time, the actual charging electrode voltage with a programmed charging electrode voltage to identify excess charging electrode decay rate, which is indicative of back ionization. The Castlet controller uses the difference in the actual and the programmed charging electrode voltage to determine a rate of application of voltage to the charging electrodes in an effort to optimize precipitator operation in the presence of back ionization.
U.S. Pat. No. 5,032,154 discloses, among other things, a system that provides automatic control of the opacity of the effluent of a coal-fired boiler to maintain minimal opacity of the flue gas effluent passing from the boiler into the atmosphere. Systems of U.S. Pat. No. 5,032,154 provide a controlled flow of an agent, such as sulfur trioxide, to the boiler flue gas to condition particulate matter entrained in the flue gas for removal by electrostatic means, monitor precipitator power and the opacity of the boiler flue gas after it leaves the electrostatic particle-removal means, and vary the controlled flow of conditioning agent to hunt and operate at desirable conditioning agent flow rates determined from flue gas opacity alone or combined with precipitator power.
Other conditioning systems are shown, for example, in U.S. Pat. Nos. 3,686,825; 3,893,828; 4,042,348; 4,284,417; 4,466,815; 4,533,364; and 4,624,685.